Jeff Mitton: Sand treaders move continually among the dunes

San Rafael sand treaders are rarely seen, for they usually forage at night. (Jeffrey Mitton / Courtesy Photo)

Jeff Mitton Natural Selections

Rain fell in the desert west of Lake Powell for several hours in the early afternoon. As soon as it stopped, I took a walk among small dunes to see what was in bloom and who was out and about.

I was more than a bit surprised to see a large, black sand treader foraging and digging in the sand. Perhaps the rain summoned the sand treader — they rarely appear before dusk.

This animal was clearly a cricket, for it had the familiar antennae for smelling and touching, the posterior structures called cerci that have tiny hairs sensitive to sounds, and a body similar to the body of a field cricket. But sand treaders have more rounded backs, shiny exoskeletons and very large back legs with many prominent spikes. Most sand treaders have light colors and subtle bands, but this one was entirely black.

This animal was a female (note the prominent ovipositor) San Rafael sand treader, Utabaenetes tanneri, and a species about which we know very little. It is the only species within its genus, so it has no close relatives, and it was only described in 1970. The only paper about it is the one that announces its discovery and describes its morphology.

The first ones were found on the sand dunes around Hanksville, Utah, so we have no idea how widely distributed it is.

To learn something about sand treaders, I read about the natural history of the giant sand treader, Daihinibaenetes giganteus, which was described in 1960 and was presented as endemic of Great Sand Dunes National Park in the San Luis Valley. Since that time, it has been found in other nearby localities, so it is no longer considered an endemic.

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The giant sand treader and the San Rafael sand treaders appear to grow to similar sizes, clearly larger than the average field cricket. The giant treader, and all others are light colored with light banding, and have large back legs with barbs or spikes that form a "sand basket" used to throw sand backwards as they dig a burrow.

The black sand treader that I was watching and following would walk one or two feet, and then bring her mouth in contact with the sand. I could see that she was sifting sand particles with her mouthparts, and I presume that she was getting something small, like mites. Sand treaders eat virtually anything, from dead plant and animal material to live plants and live animals that they can overcome. They also cannibalize moribund and dead sand treaders.

How does a species live in tunnels dug into something that is constantly moving? After all, they live in sand dunes and winds build and move dunes constantly.

A study of individually marked giant sand treaders at Great Sand Dunes provided the data on construction and use of burrows and daily movements. Treaders most prefer to place burrows in moist, cohesive sand, for it is stable and easy to dig, but when the sand dries the tunnel may collapse. Consequently, the majority of treaders dig a new burrow each day, just before dawn. One treader was recorded living in the same burrow for three days, but that was clearly unusual.

Adults dig burrows 31 to 92 centimeters long, usually at a shallow angle to the surface of the sand, and some burrows reach a depth of 45 centimeters below the surface. Burrows can be straight, slightly curved, or branched.

On any night, solitary males or females occupy about 45 percent of the burrows, and another 45 percent are occupied by a male and a female. A male defending a harem of two to four females occupies the remaining 10 percent. But mating alliances are no more stable than the shifting sands. A female in a harem one night might be with a different male the next night, and then alone the following night. On consecutive nights, a treader's burrows are commonly 50 meters apart and occasionally 200 meters apart. Treaders are always on the move.

A casual visit to Great Sand Dunes might give the impression that only a few plants and tiger beetles occupy the dunes. But mark and recapture experiments estimate that 33,000 giant sand treaders are digging new burrows each night.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado

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